The present invention relates to a semiconductor device which is used in a solid-state image sensing device or an electro-photographic photosensitive body and is intended to sense light.
In this specification, the term "light" is defined to mean the electro-magnetic waves falling between an ultraviolet-ray region and a gamma-ray region.
In an electronic copying machine, for example, the semiconductor device used in an electro-photographic photosensitive body must satisfy the following two requirements: first, it must have photoconduction; and, secondly it must hold, for a prescribed period of time, the electrical charge produced, due to a corona discharge, on its photosensitive body surface. Thus, the electro-photographic photosensitive body should have the electrical properties of being high in dark resistance or resistivity (approximately 10.sup.14 .OMEGA.cm) and of becoming low in resistivity when irradiated with light.
The principle of the electronic copying machine will now be explained briefly, in clarifying the above-mentioned requirements to be satisfied by the semiconductor device. Corona discharge is so effected that an electrical charge may flow onto the photosensitive body surface, thereby electrically charging the same. The photosensitive body surface holds that electrical charge therein for a predetermined period of time. Accordingly, the photosensitive body must have high dark resistance. Thereafter, light is irradiated onto the photosensitive body. Consequently, paired carriers of electrons and holes are produced on the photoconductive layer. Either one of these electron-hole pairs will neutralize the electrical charge held on the photoconductive surface, while the other thereof flows toward a conductive base member. Accordingly, the photosensitive body must have low resistivity when light is irradiated thereonto. For example, if the surface of the photosensitive body is positively charged, the electron produced upon the irradiation of light neutralizes the resultant electrical charge resulting the positive charge, and the hole produced will flow toward the conductive base member. Specifically, the latent image of an electrostatic charge is formed on the surface of the photosensitive body, due to the irradiation of light. Thereafter; the toner, which is so charged that its electrical charge may have a negative or positive value different from that of the electrical charge forming the latent image on the photosensitive body surface, is adhered thereto in accordance with Coulomb's law. Finally, this toner is transferred onto a sheet of paper, thereby completing the photographic copy.
In order to satisfy the two requirements referred to above, the semiconductor device used in the prior art electro-photographic photosensitive body is comprised of a base member having conductivity; an insulator or a semiconductor layer of one type formed on the conductive base member and having high resistivity; a photoconductive layer formed on the semiconductor layer or on the insulator; and a photosensitive layer formed on the photoconductive layer and having a photosensitive surface. In this prior art semiconductor device, the flow of an electrical charge from the conductive base member into the photosensitive layer is prevented by the action of the insulator or of the semiconductor layer of one type, and this electrical charge is held in the photosensitive surface for a predetermined period of time. When the single semiconductor layer is employed, the flow of a selected electrical charge (e.g., a positive charge) from the base member into the photosensitive layer cannot be fully prevented. When the insulator is employed, the flow of the selected electrical charge from the base member into the photosensitive layer can be reliably prevented. However, at the same time, the flow of the electrical charge of the opposite type (e.g., a negative charge) from the photosensitive layer to the base member is also prevented. In this case, the residual voltage will increase, resulting in fog. In this prior art semiconductor device, for example three layers are formed or stacked on the conductive base member, i.e., an amorphous silicon layer doped with boron (B) and carbon (C) and serving as a semiconductor layer, an amorphous silicon layer doped with a little of boron (B) and serving as a photoconductive layer, and an amorphous silicon layer doped with carbon (C) and serving as a surface layer, in the order mentioned. This prior art electro-photographic photosensitive device is capable of being charged with an electrical charge of approximately 300 V.
The above-mentioned prior art semiconductor device does not have the capacity to hold an electrical charge on its photosensitive layer for a specified period of time, after this electrical charge is charged therein, i.e., it does not have a satisfactory percentage of potential maintenance. For example, the percentage of potential maintenance was approximately 40% when 15 seconds had elapsed, after the device was charged.